Fix PR creation via api between branches of same repo with head field namespaced (#26986) (#29857)

Backport #26986 by @norohind

Fix #20175

Current implementation of API does not allow creating pull requests
between branches of the same
repo when you specify *namespace* (owner of the repo) in `head` field in
http request body.

---------

Co-authored-by: norohind <60548839+norohind@users.noreply.github.com>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

routers/api/v1/repo/pull.go

@@ -969,6 +969,8 @@ func parseCompareInfo(ctx *context.APIContext, form api.CreatePullRequestOption)
 			return nil, nil, nil, nil, "", ""
 		}
 		headBranch = headInfos[1]
+		// The head repository can also point to the same repo
+		isSameRepo = ctx.Repo.Owner.ID == headUser.ID
 
 	} else {
 		ctx.NotFound()